In recent years, research and development of a high density optical disc system capable of recording and/or reproducing information (hereinafter, “record and/or reproduce” will be referred as “record/reproduce”) by using a blue-violet semiconductor laser with a wavelength of about 400 nm, are advancing rapidly. As an example, in the case of an optical disc on which information is recorded and/or reproduced under the specifications that NA is 0.85 and a light source wavelength is 405 nm, namely, in the case of the so-called Blu-ray Disc (hereinafter, BD), it is possible to record information of 25 GB per layer for an optical disc with a diameter of 12 cm, which is same in size as DVD (NA is 0.6, wavelength of a light source is 650 nm, and memory capacity is 4.7 GB).
In the meantime, a value as a product for an optical disc player/recorder (optical information recording and reproducing apparatus) is not sufficient when the optical disc player/recorder only can record and/or reproduce information properly for the high density disc of this kind. In view of the realities that DVDs and CDs (Compact Discs) on which various types of information are recorded are on the market at present, only conducting information recording and/or information reproducing for the high density optical disc is not sufficient, and ability to conduct information recording and/or information reproducing properly also for DVDs and CDs owned by users, for example, enhances commercial value of the optical disc player/recorder for the high density optical disc. With the aforesaid background, an optical pickup device to be built in the optical disc player/recorder for a high density optical disc is requested to have capability to conduct information recording and/or information reproducing properly for any of high density optical discs, DVDs, and CDs, while maintaining compatibility.
As a method which enables to record and/or reproduce information adequately for any of high density optical discs and DVDs and further for CDs with maintaining compatibility, there can be considered a method to selectively switch an optical system for high density optical discs and an optical system for DVDs and CDs, corresponding to the recording density of an optical disc on which information will be recorded and/or reproduced. However, it is disadvantageous for the size-reduction and increases the cost, because it requires a plurality of optical systems.
Accordingly, in order to simplify the structure of an optical pickup device and to intend the reduction of cost, it is preferable to make an optical system for high density optical discs and an optical system for DVDs and CDs into a common optical system, and to reduce the number of optical parts constructing the optical pickup device as much as possible, even in the optical pickup device with compatibility. Then, providing the common objective optical element which is arranged with facing an optical disc, is most advantageous for simplification of the construction and cost reduction of the optical pickup device. Here, in order to obtain an objective optical element which can be commonly used for plural kinds of optical discs for which different recording/reproducing wavelengths are used, it is required that a diffractive structure having a wavelength dependency in spherical aberration, is formed in the objective optical system, to reduce spherical aberrations caused by a difference in wavelength and a difference in thickness of protective layers.
Patent Literature 1 discloses an objective optical element for recording and/or reproducing information compatibly for high density optical discs and conventional DVDs and CDs.